Silicon carbide (SiC) is a wide-bandgap semiconductor having a wide forbidden bandgap of from 2.2 eV to 3.3 eV. Because of its excellent physical and chemical characteristics, research and development of SiC as an environment-resistant semiconductor material have been performed. Particularly in recent years, SiC has been attracting attention as a material for, for example, a device for short-wavelength light ranging from blue to ultraviolet, a high-frequency electronic device, or a high-voltage and high-output electronic device, such as a power semiconductor. Research and development have been actively performed on production of a device (semiconductor device) based on SiC.
In promoting practical use of the SiC device, it is indispensable to produce a large-diameter SiC single crystal. In many of such cases, a method involving growing a bulk SiC single crystal by a sublimation recrystallization method, which is called a modified Lely method, is adopted. That is, crystal powder of SiC is placed in a crucible, a seed crystal formed of a SiC single crystal is attached onto a lid of the crucible, and the SiC crystal powder is sublimated, to thereby grow a SiC single crystal on the seed crystal. Thus, an ingot SiC single crystal having a substantially cylindrical shape is obtained. After that, the ingot SiC single crystal is generally cut into a thin plate shape having a thickness of from about 300 μm to about 600 μm using a multi-wire saw or the like and is subjected to any of various kinds of polishing treatment to produce a SiC single crystal substrate.
In the case of the sublimation recrystallization method, a crystal is generally grown under a state in which a temperature gradient is formed so that the temperature is lower on a seed crystal side as compared to a SiC crystal powder side serving as a raw material for a grown crystal. In this case, the inside of a growth space is controlled to form an isotherm having an appropriate convex shape toward a growth direction, so that a good-quality SiC single crystal ingot may be obtained. However, such temperature difference in the growth space also causes a thermal stress to remain in the SiC single crystal thus grown. Such thermal stress varies for each ingot obtained, and hence may generate a crack (cracking) in the ingot as an accidental trouble during cutting with a multi-wire saw, for example.
In addition, in production of the SiC single crystal substrate, a processing strain is formed by cutting of a thin plate-like SiC single crystal out of the SiC single crystal ingot and polishing thereof. The processing strain appears as warpage of the SiC single crystal substrate, and such warpage may pose a significant problem, such as causing defocus in an exposure process during device production. Accordingly, in ordinary cases, the processing strain is removed by diamond polishing, chemical mechanical polishing (CMP), or the like. However, if the processing strain remains or the SiC single crystal substrate has a thermal stress as an internal stress, when an epitaxial film is formed for use in a device production application, the resultant SiC single crystal epitaxial wafer may warp.
As a measure against those problems, there is known a method involving annealing the SiC single crystal ingot or the SiC single crystal substrate at a high temperature around 2,000° C., to thereby remove the thermal stress or the processing strain remaining in the silicon carbide single crystal (see, for example, Patent Literatures 1 and 2). However, no method of efficiently evaluating the thermal stress or the processing strain in the SiC single crystal has been known heretofore.
For example, through precise measurement of lattice constants using an X-ray, it is technically possible to evaluate a strain of a lattice of a single crystal. However, the measurement is not suited for industrial utilization because of, for example, the following reasons: the measurement requires expensive equipment and high skills for its implementation, and the measurement requires a long period of time.
Meanwhile, for a gallium nitride-based semiconductor light-emitting device having a laminated structure in which an n-type GaN layer, a light-emitting layer, and a p-type GaN layer are arranged on a sapphire substrate, there is a disclosure of a method of evaluating an average lattice strain amount of the GaN layer over all the layers of the laminated structure through use of Raman spectroscopy (see Patent Literature 3). In this method, a Raman shift is measured in such a manner that excitation light reaches all the layers in the laminated structure, and the measured value is converted to an a-axis lattice strain amount of the GaN layer on the basis of a known value.
However, in the case of Patent Literature 3, evaluation based on the Raman shift can be performed probably because a lattice strain is generated due to differences in lattice constants between the sapphire substrate and GaN, and also due to a tensile stress resulting from differences in lattice constants among the layers including the n-type GaN layer, the light-emitting layer, and the p-type GaN layer. That is, a difference (Raman shift) between a frequency of Raman-scattered light and a frequency of incident light is very slight in the first place, and hence a lattice strain of a SiC single crystal alone is difficult to evaluate. In addition to Patent Literature 3, there is a report of an example in which fine LSI obtained by bonding a dissimilar metal onto a Si substrate is measured for stress distribution in the vicinity of a titanium silicide pattern in the fine LSI, and in the vicinity of an element isolation film through use of micro-Raman spectroscopy (see Non Patent Literature 1). However, it is disclosed that a large stress of from 150 MPa up to 350 MPa acts on the Si substrate in the vicinity of the titanium silicide pattern. Accordingly, the measurement is performed under a state in which a large lattice strain is induced after all.
Besides, the Raman shift is liable to be subjected to influences of fluctuation in wavelength of laser light of a Raman spectroscope to be used for measurement and a thermal strain of the measuring instrument, and hence reproducibility of its measurement is not sufficient. Accordingly, even when it is in principle or in investigatory terms possible to measure the Raman shift, the measurement is not suited for evaluating a lattice strain of a single crystal alone in an industrial production application.